1. Field of the Invention
The invention deals with the field of power plant technology. It relates to an axial-flow thermal turbomachine which has a reduced rotor weight compared to the known prior art.
2. Discussion of Background
Thermal turbomachines, e.g. high-pressure compressors for gas turbines or turbines, substantially comprise a rotor fitted with rotor blades and a stator, in which guide vanes are mounted. The rotor blades and guide vanes each have a main blade section and a blade root. To enable the blades and vanes to be secured to the rotor or in the stator, grooves are formed in the stator and on the rotor shaft. The roots of the guide vanes and rotor blades are pushed into these grooves and then held in place.
The stationary guide vanes serve the purpose of diverting the flow of the gaseous medium which is to be compressed or expanded onto the rotating rotor blades in such a way that the energy is converted with optimum efficiency.
It is known to produce blades and vanes integrally from a single material, e.g. from stainless steel for gas turbine compressors or from a nickel-base superalloy for gas turbines and to use these identical blades or vanes to produce a row of blades or vanes. Blades or vanes of this type are referred to below as conventional blades.
For certain applications, the mean mass of a row of blades is limited by the load-bearing capacity of the rotor.
Therefore, there are known solutions for producing blades in a hybrid form. In the case of the hybrid form, different materials with different physical properties are combined with one another to produce a blade in order to obtain an optimum blade design. For example, a hybrid rotor blade for an engine, in which the trailing edge of the main blade section, which has only an aerodynamic function, is made of a lightweight material, preferably a fiber composite material, e.g. carbon fiber composite material, is known from DE 101 10 102 A1. A (lightweight) trailing edge of this type advantageously makes it possible to reduce the weight of the blade. The two parts of the main blade section (heavy metallic leading edge and lightweight trailing edge made of fiber composite material) are joined by adhesive bonding or riveting.
A similar solution is described in WO 99/27234, which discloses a rotor with integral blading, in particular for an engine, on the circumference of which rotor blades are arranged, the rotor blades, in order to reduce vibrations, having a metallic blade root, a metallic main blade section, which forms at least part of the blade leading edge and of the adjoining region of the blade surface, and a main blade section made of fiber-reinforced plastic. In this case too, the main blade section made of plastic is secured to the metallic part of the main blade section by adhesive bonding/riveting or by means of a clamp fit.
This known prior art has the drawbacks listed below. Firstly, the abovementioned forms of attachment are unable to withstand high loads over the course of a prolonged period of time, and secondly the fiber-reinforced plastics can only be used in certain temperature ranges, and consequently these known technical solutions are only suitable in particular for engine technology. Moreover, the characteristics of the main blade section (mechanical properties, resistance to oxidation, frictional properties) are altered compared to those of the main blade sections which consist of a single material, and this can have an adverse effect on the operating performance of the engine.
Furthermore, EP 0 513 407 B1 has disclosed a turbine blade made of an alloy based on a dopant-containing gamma-titanium aluminide, which comprises main blade section, blade root and if appropriate blade covering strip. During production of this blade, the casting is partially heat-treated and hot-formed in such a manner that the main blade section then has a course-grained structure, which leads to a high tensile strength and creep rupture strength, and that the blade root and/or the blade cover strip has a fine-grain structure, which leads to an increased ductility compared to the main blade section.
Although the use of these blades consisting of gamma-titanium aluminide advantageously reduces the mass of the rotor compared to conventional blades, a drawback of this prior art is that the blade tips, on account of their brittleness, flake off when they come into contact with the stator during operation. However, it is not normally possible to prevent this friction.
Experience gained with steel blades in high-pressure compressors has shown that even with what are known as abradable layers on the stator, the blade tips of the rotor blades can become ground down while the compressor is operating. This entails a considerable frictional force, which leads to brittle blade fracture if the blade is not ductile.